First Things First. Newton s First Law

Transcription

1 First Things First Newton s First Law

2 Quick Review In the last unit, we learned how motion can be described With words With graphs With diagrams With numbers and directions

3 Intro In this unit, we ll learn how to EXPLAIN motion. We all have an intuition about how objects move. It s part of our brains instinct. Our beliefs are much like those written down by Aristotle 2400 years ago, in his book titled Physics.

4 Our beliefs are hard to change since they work well in our everyday lives. But they limit us in understanding how the world works. We must build on our intuition and move beyond it to actually understand motion. That is just what Galileo did 400 years ago.

5 Galileo Galilei Conducted actual experiments not philosophy Believed in God, but critical of Italian church dogma

6 ARISTOTLE (and us) In our experience, objects must be pushed in order to keep moving. So force would be needed to have a constant velocity. GALILEO Galileo proposed the following thought experiment. It demonstrated another perspective This is what Aristotle claimed.

7 Imagine 2 perfectly smooth ramps connected together by a perfectly smooth surface. If a ball rolls down one ramp, it keeps rolling up the other side until it reaches the same height. (Remember PE and KE?)

8 Now repeat that experiment, but make the second ramp less steep. The ball will still keep rolling until it reaches the same height, but is has to roll farther.

9 Finally, make the second ramp flat. Now what will happen?

10 Galileo vs. Aristotle It is not that Aristotle was wrong. In everyday life, objects DO need to keep being pushed in order to keep moving. (e.g. Push a book across a table. When you stop pushing, the book stops moving.) Aristotle is right in terms of what we see around us every day.

11 Galileo defined inertia Galileo reasoned that moving objects eventually stop because of a force he called FRICTION. If there were no friction, objects would keep moving forever.

12 Galileo to Newton Isaac Newton built on Galileo s thoughts about motion. Newton s 1 st Law declares that a force is NOT needed to keep an object in motion, A force is required to STOP the motion.

13 Who is this Newton guy? Lived in the 1600s Originally wanted to be a priest Cambridge University Was Knighted and given the title of Sir Buried with royalty Lived and worked alone never married

14 Newton s Contributions Besides the laws of motion, he described Calculus Light is composed of rainbow colors Reflecting Telescope Theory of Gravitation ONE SMART COOKIE!

15 Galileo s principle was to become Newton s First Law (law of inertia) Every object continues in its state of rest or in its state of constant velocity unless acted on by an unbalanced force.

16 What is an unbalanced force? This is a balanced force which means that the sled will not move. If the force were unbalanced (like having more dogs on one side) the sled would be pulled in the direction of the bigger force.

17 What s an unbalanced force? Have you ever been in an egg race? Suppose you were carrying an egg on a spoon and walking around the track at Orion, walking as quickly as possible.

18 What s an unbalanced force? The egg would have a tendency to fall off during specific locations on the track. The egg will topple when: The egg is at rest and you start to move You and the egg are moving then stop quickly You are moving then change direction as you walk around the curve

19 What s an unbalanced force? The egg topples whenever the STATE OF MOTION is changed. The egg wants to keep doing what it was doing it s LAZY When you START walking, the egg topples backward because it wants to stay where it was. When you STOP walking the egg topples forward because it wants to keep moving on the same path.

20 What is inertia? Inertia is the resistance an object has to a change in its state of motion

21 Inertia Fail As the truck brakes, the unbalanced forces of the road and the brakes make the truck stop. There are no unbalanced forces on the stone so it wants to keep moving in a straight line.

22 If objects in motion tend to stay in motion, why don t moving objects keep moving forever? Things don t keep moving forever because there s almost always an unbalanced force acting upon them.

23 So what is a force? Force is the generic name for a push or a pull on an object. It results from an interaction between 2 objects. Force is a VECTOR. It has both magnitude and direction. The metric unit of force is the Newton (N) The English unit of force is the Pound (lb)

24 Hint: Weight is another name for the force of gravity.

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26 If you can t view the video, go to

27 INERTIA is not a force!! It is a property of matter.

28 And now, to scar your brain forever

29 Quiz Question 1 1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place and throw a rock. The rock will a. Gradually stop b. Continue in motion but curve down c. Continue in motion in a straight line.

30 Answer 1 According to Newton s 1 st Law, the rock will continue in motion in the same direction at constant speed. The answer is c

31 Quiz Question 2 A 2 kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

32 Answer 2 An object in motion will maintain its state of motion (going the same speed in the same direction.) The presence of an unbalanced force changes the velocity of the object. The answer is 0 Newtons required to maintain motion.

33 Quiz Question 3 Supposing you were in space in a weightless environment, would it require a force to set an object in motion? Explain.

34 Answer 3 Even in space objects have mass. If they have mass, they have inertia and they resist changes in motion. A force must be applied to an object to make it move. Newton s laws rule even in space! The answer is yes, a force would be required to set an object in motion.

35 Quiz Question 4 A group of physics teachers is playing mini-golf at Toads. The 7 th hole has a large metal rim that putters must use to guide their ball toward the hole. When the ball leaves the rim, which path will it follow?

36 Answer 4 Once leaving the rim, the ball will follow an inertial path (a straight line.) At the instant shown in the diagram, the ball is moving to the right. Once leaving the rim, there are no more unbalanced forces (the rim is gone) to change its state of motion. Paths 1 and 3 both show the ball continually changing it direction once leaving the rim. The answer is 2

37 Quiz Question 5 A 4.0 kg object is moving across a friction-free surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion? a. 0 N b. 0.5 N c. 2.0 N d. 8.0 N e. Depends on the speed

38 Answer 5 If an object is in motion, then it will stay in motion with the same speed and same direction. It doesn t take any force to maintain that same state of motion. In fact, the presence of a force would ruin such a state of motion and cause an acceleration. The answer is a 0 Newtons

39 You re Done with the PPT! Create a new line on your Notebook Scorecard titled Newton s First Law PPT. Award yourself 5 points for taking notes and 1 point for each correct quiz question. (max 10 points)

40 Practice this new knowledge of Newton s First Law by completing a worksheet. What you do not finish in class becomes homework.